Triazolyl–Based Copper–Molybdate Hybrids: From Composition Space Diagram to Magnetism and Catalytic Performance

The multicomponent
mixed-metal CuII/MoVI oxides/1,3-bis­(1,2,4-triazol-4-yl)­adamantane
(tr2ad) system was thoroughly
studied employing a compositional diagram approach. The concept allowed
us to prepare three layered copper–molybdate hybrid solids
[CuII2(tr2ad)4]­(Mo8O26) (1), [Cu4II(μ4-O)­(tr2ad)2(MoO4)3]·7.5H2O (2), and [CuI2(tr2ad)2]­(Mo2O7)·H2O (3), and to elucidate the relationship between initial
reagent concentration/stoichiometry and the stability of the resultant
structural motifs. Compounds 1 and 2 were
found to dominate throughout a wide crystallization range of the concentration
triangle, whereas compound 3 was formed by redox processes
in the narrow crystallization area having a high excess of Cu­(OAc)2·H2O. Independent experiments carried out
with Cu­(OAc)2 and (NH4)6Mo7O24 in the absence of tr2ad, under the same conditions, revealed the formation of
low-valent and bimetallic oxides, including Cu2O, MoO2, Cu­(Mo3O10)·H2O, and
Cu3(MoO4)2(OH)2. Compounds 1 and 2 show high thermal and chemical stability
as examined as catalysts in the epoxidation of cis-cyclooctene and the oxidation of benzyl alcohol (BzOH) with different
types of oxidants. The oxidation reaction of BzOH using tert-butyl hydroperoxide (TBHP) as the oxidant, in the presence of 1 or 2, led to benzaldehyde and benzoic acid
(PhCO2H), with the latter being formed in up to 90% yield
at 24 h. The results suggest that 1 and 2 may be favorable heterogeneous catalysts for the synthesis of PhCO2H. Whereas compound 1 only reveals a weak ferromagnetic
coupling between neighboring CuII centers (J = 0.41 cm–1), compound 2 shows distinct
intracluster antiferromagnetic exchange interactions (J = −29.9 cm–1, J′
= −25.7 cm–1), which consequently results
in a diamagnetic ground state.